1. Field of the Invention
This invention relates generally to siting aids, and more particularly to a siting system particularly useful for precisely locating a portable ground station or localizer for an azimuth guidance system for aircraft.
Various azimuth guidance systems and ways to site those systems are known In a typical azimuth approach system, a ground station is employed to transmit a beam containing azimuth information to provide guidance to a pilot of an approaching aircraft Such azimuth guidance systems utilize ground stations that may be located at either the take off end of the runway or at the approach end of the runway near the touchdown point. In either case, it is important accurately to orient the beam or boresight of the ground station antenna to assure accurate guidance information. When the ground station is located at the take off end of the runway, the location of the ground station and the proper orientation of the antenna can readily be determined because the station is located along the center line of the runway and its antenna is pointed so that a boresight located on the antenna parallel to the center of the antenna beam follows the center line of the runway. Thus, the location of the station and the orientation of the antenna can be readily accomplished by sighting along the runway through a telescope or transit.
Alternatively, the ground station can be located near the approach end of the runway at a point adjacent to the runway near the touchdown point. A system having the ground station or localizer thus located is called a collocated system. Because the station is not located on the center line of the runway, if the antenna were pointed in a direction so that its beam ran parallel to the runway, the pilot would be guided to a location adjacent the runway. Consequently, the antenna is angled toward the runway so that the beam intersects an extended center line of the runway at a a predetermined point ahead of the touchdown point, typically 3816 feet. At this point the aircraft should be at its decision height (DH). The decision height is defined as a specified height at which a missed approach must be initiated if the required visual reference to continue the approach to land has not been established. The term required visual reference means that a section of the visual aids or of the approach area should have been in view for a sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path. The angle that the boresight of the antenna is offset relative to a line parallel to the runway center line is called the pointing angle.
One way to orient the antenna of the ground station so that the beam intersects the extended runway center line at the point and utilize a telescope or a boresight located on the antenna to sight the person or marker through the telescope or boresight to determine the proper orientation of the antenna. While this solution may be satisfactory for civilian installation where access to the decision height point is available or for fixed installations where the orientation of the antenna is seldom changed, it is not a very satisfactory solution for portable installations, particularly military installations wherein the sites are often moved and the decision height point may not be accessible.
Thus, for such installations, the pointing angle must be calculated indirectly. One way to do so is to utilize a magnetic compass to determine the heading at various points and to calculate the pointing angle from the differences in heading. However, magnetic compasses do not provide the accuracy required, are effected by magnetic material in the vicinity and are unusable in polar latitudes.
Another method is to measure distances to predetermined points along the runway, place stakes into the ground at those points, and use triangulation techniques to determine the appropriate pointing angle. However, while such a technique provides accurate results, the distances that must be measured to provide the required accuracy require the use of very long tape measures on the order of 300 feet long. Unfortunately, such tape measures are very heavy and their weight is a disadvantage, particularly in portable applications.